1. Field of the Invention
The present invention relates to an image display apparatus such as a head mounted display which displays an original image formed in a single image forming device to an observer.
2. Description of Related Art
Image display apparatuses of a head mounted type (so-called head mounted displays) have conventionally been known in which an image forming device such as an LCD (Liquid Crystal Display) is used to enlargingly display an original image displayed in the image forming device through an optical system.
Since the head mounted display is mounted on the head of an observer, reductions in size and weight of the entire apparatus are particularly needed. The display preferably has a small thickness in the optic axis direction of the observer in consideration of weight balance, appearance and the like. In addition, it is desirable to display the largest possible enlarged image to produce a dynamic effect in the image.
For example, each of Japanese Patent Application Laid-Open No. H7 (1995)-333551, Japanese Patent Application Laid-Open No. H8 (1996)-50256, Japanese Patent Application Laid-Open No. H8 (1996)-160340, and Japanese Patent Application Laid-Open No. H8 (1996)-179238 has proposed a head mounted display which uses an LCD as an image forming device and a thin prism as an observation optical system to achieve a reduction in thickness of the entire apparatus.
FIG. 16 shows a head mounted display proposed in Japanese Patent Application Laid-Open No. H7 (1995)-333551. In the head mounted display, light emitted from an LCD 111 is incident on an incident surface 113 of a small decentered prism 112. The luminous flux is folded by a reflective surface 114 and a reflective surface 115 each having a curvature and formed on the prism 112, and then emerges from the surface 114 of the decentered prism 112 and is guided to an eye E of an observer. In this manner, a virtual image of an original image displayed in the LCD 111 is formed and observed by the observer.
The reflective surface 115 of the decentered prism 112 is formed of a decentered free-form surface made of a decentered rotationally asymmetric surface (a surface exhibiting different optical power depending on an azimuth angle, or a so-called free-form surface).
The optical system of the type shown in FIG. 16 is characterized by easily realizing a reduced thickness of the entire apparatus and a larger field angle of the visual field for observation as compared with a conventional type which uses a coaxial concave mirror and a half mirror inclined 45 degrees with respect to the optical axis of an eyeball.
In addition, it is desirable that the head mounted display is provided at a low price similarly to other image display apparatuses. As a solution therefor, U.S. Pat. No. 4,322,135 has disclosed a head mounted display of a single original image binocular observation type in which a single image forming device is used in combination with an optical system which guides a single original image formed in the image forming device to each of the left and right eyes of an observer.
FIG. 17 shows the structure of the head mounted display disclosed in U.S. Pat. No. 4,322,135. Image light emitted from a convex original image forming surface 202 of an object source 201 for displaying an original image such as an image intensifier is incident on an optical element 204 from its transmission area 205, reflected by portions 208 each having a reflective film formed thereon or internally reflected by transmission areas 207 of the optical element 204, reflected by concave mirror portions 203 each having a reflective film formed thereon, and then emerges from the transmissive areas 207. The light emerging from the optical element 204 passes through meniscus lenses 210 and is guided to both eyes E1 and E2 of an observer.
In addition, various head mounted displays have been proposed in recent years in which the head mounted display of the single original image binocular observation type is used in combination with a prism having a decentered free-form surface as described above. For example, Japanese Patent Application Laid-Open No. H9 (1997)-61748, Japanese Patent Application Laid-Open No. H9 (1997)-247579, Japanese Patent Application Laid-Open No. 2000-177785, Japanese Patent Application Laid-Open No. 2001-194618, and Japanese Patent Application Laid-Open No. 2001-194619 have proposed the structure of such displays.
The conventional head mounted displays described above, however, for example the displays proposed in Japanese Patent Application Laid-Open No. H9 (1997)-61748 and Japanese Patent Application Laid-Open No. H9 (1997)-247579, involve a high cost since different optical systems are used for both eyes.
In the other examples, the single optical system can be shared between both eyes. However, when the light from the image forming device to the pupil of image light is inversely traced from the pupil side, the inversely traced light is reflected by the first reflective surface inward toward the center laterally, so that it is difficult to realize a larger field angle.
The disadvantage is now described with reference to FIG. 18. Light emerging from an image forming device 311 is incident on an incident surface 313L of a small decentered prism 312L. The light is folded by a reflective surface 314L and a reflective surface 315L each having a curvature and formed on the prism 312L, and then emerges from the surface 314L of the decentered prism 312L and is guided to a left eye EL of an observer.
Similarly, light emerging from the image forming device 311 is incident on an incident surface 313R of a small decentered prism 312R. The light is folded by a reflective surface 314R and a reflective surface 315R each having a curvature and formed on the prism 312R, and then emerges from the surface 314R of the decentered prism 312R and is guided to a right eye ER of the observer.
In this manner, a virtual image of a single original image displayed in the image forming device (an LCD) 311 is observed by the left and right eyes EL and ER of the observer.
The following description is made with inversely traced light which is frequently used in optical design in the virtual image observation system. Rays from the eyes of the observer (rays from the pupil) are incident on the prisms 312L and 312R from the surfaces 314L and 314R. The rays are folded by the reflective surfaces 315L and 315R serving as the first decentered reflective surfaces in this case to approach the normal to the image forming device 311 displaced substantially at the center of both eyes. Thus, in the optical system described above, reflective areas shown by EA1L and EA1R are necessary in the surfaces 314L and 314R of the prisms 312L and 312R. These reflective areas EA1L and EA1R become larger as the field angle is larger.
The distance between the left and right eyes EL and ER (interpupillary distance) of an observer, however, is determined to some extent, so that it is impossible to extremely increase an interpupillary distance IPD between the pupils of the optical systems for left and right eyes. Consequently, a significant increase in field angle cannot be expected in this structure in which the inversely traced light is folded by the first reflective surface inward toward the center laterally.